In many situations, users of optical instruments wish to line their instruments with a material that reflects very little or no light, so that stray light which might reduce contrast or produce ghost patterns will be absorbed and not interfere with the intended operation of the instruments. In some instances glossy black paint is useful, but it reflects at least 4% of incident light in a specular manner that may produce ghost patterns. Matte black paint also reflects about 4% of incident light, in this case diffusely, so that contrast is reduced. Coatings of extremely fine conducting powders such as lampblack or platinum black often reflect only an imperceptible amount of light, but such coatings are extremely fragile and cannot be touched or cleaned. A different type of material, black velvet has some of the properties desired.
Black velvet is an example of a type of material that owes its deep black appearance to its structure. The fibers that form the pile of the velvet consist of smooth parallel cylinders of rayon, for instance. A ray of light striking a black fiber cylinder at an angle is mostly absorbed, but a few percent of the ray is reflected farther into the pile. There it strikes another fiber and again, it is mostly absorbed. The small amount not absorbed is reflected farther into the pile, strikes another fiber, and so forth, until essentially all of the light is absorbed. Black velvet is not truly ultrablack because incident rays parallel to the pile fibers will strike the support material and a small percentage of the light will be reflected back out. Also, rays that are incident upon the ends of fibers, or upon bent fibers, will be partially reflected, and provide the characteristic sheen of black velvet.
A partial reduction in the light reflected from a picture surface can be made by causing the light to undergo two surface reflections before it reaches the viewer's eye. This approach is used by Whitehead in his "Non-Reflective Graphic Surface Display Device" of U.S. Pat. No. 4,883,341. This device consists of a transparent plate grooved on the top and having its bottom surface in optical contact with the graphic surface being displayed, so that reflections from the graphic surface are eliminated and only reflections from the grooved surface remain. An ultrablack article is not suggested in the Whitehead patent.